The great escape: how cationic polyplexes overcome the endosomal barrier

The targeted and efficiency-oriented delivery of (therapeutic) nucleic acids raises hope for successful gene therapy, i.e., for the local and individual treatment of acquired and inherited genetic disorders. Despite promising achievements in the field of polymer-mediated gene delivery, the efficiency of the non-viral vectors remains orders of magnitude lower than viral-mediated ones. Several obstacles on the molecular and cellular level along the gene delivery process were identified, starting from the design and formulation of the nano-sized carriers up to the targeted release to their site of action. In particular, the efficient escape from endo-lysosomal compartments was demonstrated to be a major barrier and its exact mechanism still remains unclear. Different hypotheses and theories of the endosomal escape were postulated. The most popular one is the so-called "proton sponge" hypothesis, claiming an escape by rupture of the endosome through osmotic swelling. It was the first effort to explain the excellent transfection efficiency of poly(ethylene imine). Moreover, it was thought that a unique mechanism based on the ability to capture protons and to buffer the endosomal pH is the basis of endosomal escape. Recent theories deal with the direct interaction of the cationic polyplex or free polymer with the exoplasmic lipid leaflet causing membrane destabilization, permeability or polymer-supported nanoscale hole formation. Both escape strategies are more related to viral-mediated escape compared to the "proton sponge" effect. This review addresses the different endosomal release theories and highlights their key mechanism.